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When you say hormones, you may be thinking of estrogen and testosterone. But these complex chemicals govern far more than puberty and sexuality.

Hormones, which are natural chemical signals, affect growth, metabolism, blood pressure, and even behavior. The majority of hormones that affect the body's processes are produced by the endocrine glands. These glands release hormones directly into the blood or lymph system. The hormones travel in the blood to tissues and organs where they can attach to specific cell sites called receptors. By attaching to receptors, hormones trigger various responses in the tissues.

Endocrine glands

These are the endocrine glands:

Pineal gland. This gland in the brain secretes melatonin, which affects reproductive development and the daily sleep/wake cycle.

Pituitary gland. This gland in the brain makes hormones that control other endocrine glands.

Thyroid gland. This gland located in the neck releases thyroid, a hormone that controls the rate of metabolism (the rate cells burn fuels from food to make energy).

Parathyroid glands. This gland also in the neck releases the parathyroid hormone which, along with a hormone from the thyroid (called calcitonin), regulates the level of calcium in the blood.

Thymus gland. This gland in the chest produces thymosin, which plays an important role in the development of the body's immune system.

Adrenal glands. These glands near the kidneys in the lower belly produce several essential hormones. They help regulate salt and water balance in the body, maintain blood pressure and heart rate, and play a part in the immune system and in sexual development (before birth).

Pancreas. This gland near the stomach produces insulin and glucagon to regulate the level of glucose (sugar).

Ovaries. These glands in the female's lower belly secrete the female hormones estrogen and progesterone, which are required for proper menstruation, strong bones, and fertility.

Testes. These glands in the male's scrotum secrete male hormones called androgens, including testosterone required for normal stamina.

Other areas of the body produce hormones. The lining of the stomach produces hormones called gastrin and ghrelin, which regulate appetite and digestion of food. Special cells in the wall of the upper chambers of the heart produce atrial natriuretic hormone, or atriopeptin to control blood volume. During pregnancy, the placenta secretes human chorionic gonadotropin, which signals the mother's ovaries to secrete hormones to maintain the pregnancy.

Important hormones

Humans have about 100 known hormones. Here are some of the more important hormones:

Growth hormone is one of many hormones secreted by the pea-sized pituitary gland at the base of the brain. It stimulates and maintains the growth of bones, muscles and other organs. Underproduction stunts growth, and may contribute to depression and sluggishness.

Insulin, made by the pancreas, mainly regulates the body's blood sugar levels. Lack of insulin or the inability to efficiently use it causes diabetes.

Thyroid hormone, from the thyroid gland in the neck, regulates metabolism, growth, and development. Deficiencies cause weight gain and sluggishness, which doctors treat with synthetic thyroid hormone replacement. Overproduction causes weight loss, rapid heartbeat, and anxiety. That can be treated by surgically removing the gland, by chemotherapy, or by radiation.

Adrenaline, or epinephrine, from the two adrenal glands, regulates blood pressure and heart rate.

Cortisol, another product of the adrenal gland, plays an important role in metabolism.

Without a doubt, the most talked-about hormones deal with sex. Not only are estrogens and androgens important in sexual development and reproduction, they also affect behavior and define male and female characteristics. The loss of estrogen during menopause can cause depression and loss of energy and can also weaken bones and increase risks of heart attack and stroke.

Some older men may experience physiologic changes from decreased production of testosterone. Experts generally agree hormone therapy can help relieve menopausal symptoms and help prevent osteoporosis in postmenopausal woman. However, hormone replacement therapy also carries increased risks, so it must be individualized.